Instant heating process with electric current application to the workpiece for high strength metal forming

ABSTRACT

A process and relevant tools for hot forming of high strength metal workpieces by means of applicating high density current to the workpiece directly and generating heat inside it by using its own electrical resistance in order to obtain desired temperature and formability at the desired moment of the forming process without requiring any external heat source or previous heating process. Temperature of the blank is measured by measuring its electrical resistance and by using linear correlation between temperature and resistance. This heating process can be applicated in several metal forming types such as high strength sheet stamping (FIG.  2 , FIG.  3 ), bending (FIG.  3 ), blow forming (FIG.  4 , FIG.  5 ) in accordance with mechanical operations of the relevant processes. High temperature rates can easily be reached and kept at the desired moment of the forming process without being effected by rapid cooling phenomena resulted by too much heat loss area/mass and heat storage capasitance of thin sheets. Ceramic tools and dies are available in these process offering electrical nonconductivity, thermal low conductivity and durability against heat. Cooling process of the formed workpiece between dies under pressure provides dimentional accuracy and increased yield strength resulted by regular elongation effect and rapid temperature decrease.

FIELD OF THE INVENTON

This invention relates to a hot stamping process and apparatus forforming sheet metal alloys with low formability at room temperature. Inparticular, this invention relates to a warm/hot sheet forming operationwith rapid pre-heating process on the press table by direct electriccurrent application to the workpiece using two electrode sets contactingat two opposite edges of the workpiece.

BACKGROUND OF THE INVENTION

In the plastic forming processes of various metal parts, heating issometimes necessary before a forming operation. In the metal deformationprocesses such as forging, extrusion, rolling etc. the workpiece isheated above its recrystallization temperature prior to subsequentforming operation and these processes are generally referred as hotworking.

In common hot forming techniques, the metal workpiece is heated in afuel-fired or electric furnace before mechanical forming operationperformed by forging, rolling, extrusion, drawing etc. During the periodin which the workpiece is removed from the furnace and placed on thepress table between the dies, a considerable amount of heat is lost fromthe workpiece. Generally, heat loss is proportional to surface area ofthe workplece. Heat is held by the mass of the original workpiece andheat loss occurs in peripheral area of the workpiece by radiant,convective and conductive means. Increase in peripheral area/mass ratioof the metal workpieces results in more rapid cooling phenomena duringhandling from furnace to forming machi ne, and thus, hot forming of suchthin metal workpieces become difficult or practically impossible in somecases. Radiant heat loss becomes dominant at high temperatures, becauseit is proportional to fourth power of the workpiece temperature, andwhile conductive heat loss is linearly proportional to temperature ofthe workpiece.

Hot forging including preheating at a furnace, handling to formingmachine and then compression forming is a widely used hot workingprocess for a long time all around the world. In such a process heatloss of the hot bulk workpiece can be kept in an acceptable level anddoes not prevent the operation.

In hot working of a sheet metal workpiece with thickness between 0.6 and3 mm such as articles used in automotive industry, peripheral area/massratio is too much and such a workpiece cannot keep its temperaturesufficiently during handling of hot blank to be placed between diesafter furnace heating. A hot blank sheet loses a considerable amount ofheat and its temperature rapidly decreases below hot working temperaturerange within a few seconds. Most of high strength alloy steels, aluminumand magnesium alloys are temperature sensitive and they are onlyformable within narrow ranges of temperature. Such a heat loss becomesparticularly severe for high strength and thin alloy sheets, and thus,subsequent hot working becomes practically impossible. Therefore, thereis not a widely used hot stamping method in use for production articlesmade from high strength alloy sheet for particularly automotiveindustry.

In practice, such a thin sheet can keep its temperature only a fewseconds for subsequent forming process. For example, in a steel blanksheet in 1100° C. temperature with 1 mm thickness, temperature decreaserate is more than 100° C./sec. Heating the workpiece to highertemperatures is not a solution, because radiant heat loss varies withfourth power of the temperature and temperature fall becomes moresevere. On the other hand, overheating may alter microstructure (grainsize, structure, elongation rate, formability, strength etc.) of theworkpiece or cause surface oxidation.

Although there are much higher strength steel and aluminum alloys,currently used stamping technology can not form such metal thin sheetsby currently used sheet stamping technology due to lack of formability.Thin sheets made of such alloys can offer very higher strength up tothree or four times more strength than of currently used sheets inautomotive production. Such metal blanks principally can only haveadequate formability in high temperature rates and within tight range.

The most important utilization area of the invention is the automotiveindustry. One of the main challenges for the today's automotive industryis “How to produce lightweight and stiff auto chassis and bodyconstruction in mass production with high quality low cost”. Stampedsheet articles consist of (app. % 50 -60) most of auto body weight.There are many weight loss programs carried out by car companies,suppliers etc all around the world in efforts to make new productiontechnologies more responsive to needs of the low fuel consuming vehiclesof tomorrow. There are many technical teams in the automotive world, incollaboration with the national labs, universities and suppliers, areworking to reduce vehicle weight as compared to today's compact andmidsize family sedans. Therefore, there is a widespread tendency to usewidespread tendency to use relatively higher strength steels and lightaluminum and magnesium alloys in the automotive industry.

From aspect of safety, energy rate that can be absorbed elasticallyduring a crash by a metal auto part until plastic deformation limit isproportional to second force of its yield strength. However, a singlepart made of relatively higher strength metal might require morestamping stages than a comparable part or the part may have to divideinto two or more pieces that are then joined together. Nevertheless,these solutions add time and cost to the manufacturing process. Thus,engineers have been trying to find other methods to replace orcomplement the conventional mechanical stamping process in order tofully realize the potential weight savings of using of higher strengthsteel and aluminum components. On the other hand, such materials causemore wrinkles and tears during manufacture and require significanttry-out modification and completion works for dies and tools requiringhigher cost, time and labor.

SUMMARY OF THE INVENTION

The main principle of the invention is to achieve both direct heating ofthe blank by current application on the press table and hot stampingoperation performed as subsequent process achieved in one place (presstable) without requiring any handling operation of the workpieceresulting severe temperature fall preventing such an hot shapingprocess. Temperature fall at the hot thin sheet during handling frompre-heating furnace to press table is so severe that it is practicallyimpossible to keep its heat sufficiently until end of the stampingprocess between two dies.

For example, steel sheet thickness, 1 mm , T=1100° C., Temperaturedecrease rate =110° C./sec Heat Energy Equation of this process:$\left. {\underset{\text{Heat rate generatedinside the Workpiece~~~~~~~~~by Current}}{{R\left( {1 + {{{}_{}^{}{}_{}^{}}\Delta \quad T}} \right)}I^{2}} = {\underset{\text{Heat rate held mass~~~of the workpiece}}{\left\lbrack {m{{}_{}^{}{}_{}^{}}{{T}/{t}}} \right\rbrack} + \underset{\text{Heat loss by~~~radiation}}{\left\{ {2{{}_{}^{}{}_{}^{}}4.96{{{}_{}^{}{}_{}^{}}\left\lbrack \left( {T_{s}/100} \right)^{4} \right.}} \right.} - \left( {T_{e}/100} \right)^{4}}} \right\} + \underset{\text{Heat loss by Convection}}{A{{}_{}^{}{}_{}^{}}\Delta \quad T}$

where R electric resistance of the workpiece (Cold), α Resistanceincrease coefficient by temperature, ΔT temperature increase of thesheet, I current, m mass of the workpiece, C Specific thermalcapacitance, A Area (one side) of the WP, T_(s) temp of the sheet, T_(e)peripheral temperature, β Convection coefficient, k radiant heattransfer coefficient.

The process ensures instant temperature rate of the hot sheet at thestamping moment by controlling principal parameters of the process suchas, current, current application time, stamping time etc within onemachine. This process can be applied in mass production of articles madefrom high strength alloy sheets for automotive industry, because wholeprocess including, heating, stamping, cooling within dies is performedin one machine within a few seconds. It's very important to preventthermal or mechanic distortions of formed article during cooling afterhot stamping. Cooling must be performed without any distortion andpreferably; formed sheet should be removed from the dies aftersufficient temperature fall. Dimensional stability and sufficientstrength (after cooling) should be ensured during removing of thestamped part. Particularly, automotive industry demands strictdimensional tolerances. This process achieves instant cooling of theworkpiece without distortion by means of cooling under pressure of colddies. The dies are kept within a previously determined temperature rangethat is fairly lower than forming temperature of the workpiece. A littleamount of heat is gained by dies by contact of the hot workpiece at eachprocess cycle. On the other hand, the dies continuously lose heatbecause their temperature will be slightly higher than room temperatureduring mass production.

The process starts with current application to the workpiece for a fewseconds and temperature of the blank sheet is reached previouslydetermined rate to provide sufficient formability characteristics in theworkpiece such as elongation rate, yield strength etc. Until thiscertain temperature rate is provided in the workpiece, the dies are notin contact with hot workpiece. At least one die is moved toward the hotsheet and sheet is stamped. Temperature of the dies is fairly lower thanhot forming temperature and slightly higher than room temnperature. Aninstant cooling process is achieved at the end of the stamping while thesheet is being completely compressed with two dies and it is veryimportant to prevent thermal or mechanical distortions in order toprovide strict dimensional tolerances.

Similar heating process is also needed in sheet bending and prototypeproduction processes if the blank sheet is made from high strength alloymetal. In bending process of such blanks, similar heating operation isapplied before bending. In prototype production with use of one die, themain principle of the invention is applied and these processes areexplained below.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF DRAWING

Four figures were prepared to explain the main process and relevantproduction tools and details.

FIG. 1 shows application of the main principle in high strength sheetstamping process and consists of a plan view (upper side) and a crosssectional view (lower side) of the press table of regarding with thisinvention.

FIG. 2 is a sectional view of the press table and includes someadditional details about stamping stages.

FIG. 3 is prepared with the aim of explain how the basic process can beapplied in bending of the high strength metal workpieces and includesrelevant forming stages in sequence and relevant tools.

FIG. 4 indicates application of basic process in a press cell type usingone die and compression of solid mixture. FIG. 4 is also comprises ofplan (upper side) and sectional (lower side) views of relevant presscell.

DETAILED DESCRIPTION OF THE INVENTION

The main principle of the invention as shown in FIG. 1 is to apply highelectric current density passing entirely blank 4 from one side toopposite side by using electrodes 3 contacting with two opposite sidesof the blank 4 at the press table therefore both instant heating andstamping processes are performed in one machine. This process andrelevant apparatus ensure hot stamping process of the sheet at apreviously determined temperature and thus, suitable elongation andyield strength rates. This process eliminates carrying time betweenpreheating and stamping processes.

Actual temperature of the hot blank can be controlled preciously bymeasuring electrical resistance change of the workpiece from beginningof heating by using linear correlation between temperature andelectrical resistance. The heating system is controlled by a controldevice measng electrical resistance and calculating actual temperature,therefore control device determines acting moment of the of the formingmechanism. In mass production if proper characteristics of above processare determined by adequate research and experiences, whole process canbe carried out with previously determined parameters without using afeedback control system. “Hot forming” term as used in this descriptionincludes suitable temperature ranges providing increase in elongationrate and formability and decrease in yield strength rates for variousmetal types and these temperature ranges for various metal alloys can beabove or below metal recrystallization temperature of these metals.

As seen in FIG. 1 Electrodes 3 are placed two opposite sides of thepress table. At first, blank 4 is placed on the press table. Electrodes3 are contacted with the blank 4 and applies high density current alongthe blank. An external current source 2 provides low voltage currentwith high current rates and two end of the current source are connectedto two electrodes 3 placed two opposite edges of the blank sheet. Duringheating process, the blank holders 1, 6 do not hold the blank and allowsits regular thermal expansion laterally in order to avoid wrinkles. Theblank holders 1, 6 are made from nonconductive materials in order toavoid short circuit during direct current application to the blanksheet. The contact pressure of the electrodes 3 is properly determinedto allow expansion of the blank 4 during heating by means of controlledsliding movement between workpiece and electrodes. On the other handboth two electrode groups 3 are slightly pulled back (with an hydraulicsystem etc.) during heating in the longitudinal direction in accordingto thermal expansion with the aim of keep flat blank surface.

Thus, the workpiece instantly (within a few seconds) reaches hightemperature degrees (app. 800-1000° C. for steel and 350-500° C. foraluminum alloys). Then the binders hold the hot blank and upper die 5 ismoved down and hot workpiece is formed. Above system ensures workpiecetemperature until contact moment of the die and workpiece. Due toforming speed of the die 5 (from first contact moment to the blank tocontact moment to other die) of the (esp. Mechanical) presses issufficiently high and most of workpiece area (As Shown in FIG. 2, asindicated by dashed lines 9) (esp. areas involving high local elongationrates) do not in contact directly with the cold die surface until end ofthe forming process, the workpiece sufficiently keeps its hightemperature within fast stamping process. Blank holder surfaces 6 can bemade of ceramic insert parts in order to isolate heat and current toavoid heat loss from workpiece to press table. Because of the rapidheating, heat loss from the blank will be fairly low thereby electricenergy will be consumed efficiently to heat workpiece directly.

If relatively slow hydraulic presses are used in such a process someadjustments in die form can be made (FIG. 2) to reduce contact areabetween hot blank and cold die surface during forming (especially inchassis and frame production including flat surfaces and rounded edges)in order to reduce heat loss until end of the process.

As shown in FIG. 2 downward facing surfaces 7 of the upper die andfacing surfaces of the lower die are designed as concave 7 formssurrounded by rounded extensions 8. As indicated in FIG. 2 by dashedlines 9 most of the workpicce area is not in contact with the diesurface 5, 7 until end of the process. Since press force acting diesurfaces will be significantly lower in such a process than that ofconventional stamping due to forming in high temperature rates of theblank, “Fragile” ceramic dies can be used conveniently. In such a casehigh density current can also be applied during forming stage due toelectrical no conductivity of the material of the dies and the binders.Additionally ceramic is resistant against heat. Heat storage capacity ofa thin metal blank is low even though it is heated to the hightemperatures. It enables to control temperature of the workpiece untilend of the process. On the other hand, the workpiece cools more slowly(than of hot stamping with metal tools) after forming resulted by lowthermal conductivity of the ceramic materials. This process can beapplied in one stamping stage or can be divided into multi hot stampingstages and additional heating-annealing processes can be achieveddepending of part geometry complexity and metal features between formingstages.

This process causes considerable increase in yield strength of thefinished parts because of two reasons. First reason is rapid coolingbetween two dies. The second one is regular elongation effect (app. %1-1.5) involved through whole workpiece area. These issues will beexplained below. As it know both rapid cooling and rapid deformation(work hardening) lead to increase in yield strength especially in steelsincluding sufficient carbon or some other suitable alloys. If the blankis formed by this process in a hydraulic press, at the end of theforming stage hot workpiece cools instantly between two dies. In highstrength alloy sheets, instant cooling leads to significant increase inyield strength. During cooling stage, formed workpiece is stretched dueto rapid temperature decrease and shrinking. While hot workpiece iscooled between upper and lower dies under pressure, any considerablechange in dimensions can not occurs in spite of rapid cooling. It meansthat a regular elongation effect occurs in whole area. Total elongationrate in unit area is sum of plastic and elastic elongation rates. Afterworkpiece is ejected from the dies, the formed part shrinks elasticallyby ratio of actual yield strength/Elasticity modulus. At the diedesigning stage, this shrinkage ratio should be considered. Deviationsin dimensions of the finished parts essentially depend on deviations ofyield strength rates of the finished parts. Because stamped sheet coolsbetween dies without any practical changes in dimensions and elasticshrinkage factor can be calculated and considered previously, thisprocess is precious, dependable and reproducible.

If this process is performed in a mechanical press formed workpiece canbe ejected from lower die by an automatic mechanism while upper die ismoving up after stamping. In contrast to cooling between dies, in thiscase formed workpiece cools and shrinks in air freely. Shrinkage ratiowill be higher than above process but it is possible to control coolingand shrinkage characteristics by means of changing press speed,workpiece temperature and ejecting mechanism speed. In this process,heating and forming systems should be controlled and operated inaccordance by the same control device. Because of the rapid heating,heat loss from the blank will be fairly low thereby electric energy willbe consumed efficiently to heat workpiece directly.

The invention can also be used in bending (FIG. 3) of high strengthalloy sheets featuring very low formability. In such an application,only a long and narrow bending line 11 is heated by a set 10,12 ofelectrodes placed two sides of the bending line. A set of apparatus asseen in FIG. 3 are used for instant heating with current and bending ofthe workpiece. These tools 10, 12, 13, 14, 15 are moved in sequence bypneumatic or hydraulic etc. system in accordance with instant heatingprocess. At first, electrodes 10 and 12 are pushed onto the workpiece 17and apply high-density current to be conducted by bending are soon assufficient temperature obtained at the bending line 11, electrode 12 ismoved up and then first bending tool 13 is moved up and down thusworkpiece is bended by about 90 degrees. A portion 16 of the electrode13 can be made as a ceramic insert with the aim of reducing heattransfer between hot area 11 and tool 13. At this moment, Part 15 ismoved ahead thereby the blank is bended by 180 degrees.

If bending of the workpiece will be achieved along a curved (e.g. bodyor door panel for automotive industry) line 11 tools of this processshould be designed in accordance with curved bending line. Similarwell-known technologies about bending and resistance welding simplify toachieve above process.

This invention can be applied in (FIG. 4) “Hot stretch sheet formingwith pressure of sand/lubricant mixture”. To product low volume and highstrength panels and frames this technology will be an attractivealternative with low tooling cost due to requiring only one die (made ofceramic or concrete) and not requiring long design time and cost. Thedie 21 is placed inside the cover of the press cell. Hydraulic pistons22 are used for opening or closing upper side side of the press cell.The blank is placed on the lower housing and situated between twoopposing electrode sets 19 and edges of the blank are in contact withelectrodes. Part 20 is used for locking of upper side of the press cell.As seen at FIG. 4 in this process the hot blank is heated directly byelectric current application and predetermined temperature is reached atthe blank sheet Sand and lubricant mixture 23 is pushed up by hydrauliccylinders 24 placed bottom of the mixture bowl and therefor heated blankflows smoothly into the die 21 under pressure of the sand &. lubricantmixture 23. In this process, solid mixture 23 is in contact directly(without any flexible membrane) with the hot blank. The main principleof the invention “Instant Heat generation in the workpiece by applyinghigh current rate” is achieved in a similar way like above processes.Two opposite sides of the blank-holders include electrodes 19 and otherparts are made of ceramic inserts. Electrodes 19 are electricallyisolated from whole mechanic apparatus by isolation member 25 . Due tothermal conductivity of the sand mixture 23 is very lower than that ofmetals, heat can be generated by current along the forming process whilethe hot blank 19 is bulging into the die 21. Sand mixture 23 (or anyother proper solid material Aluminum Oxide etc.) is very durable againstheat and features very low thermal conductivity. Therefore, heatgenerated in the workpiece will not be absorbed instantly by the sand.If the die is made of ceramic or concrete, heat loss of the workpiecebecame moderate after contact moment between hot sheet and the die.

While the invention has been described in terms of a few specificembodiments thereof, many changes and other applications of theinvention will became apparent to those skilled in the art afterconsidering the specification together with the accompanying drawings.

I claim:
 1. A process for forming sheet metal workpieces before which atleast two electrode sets placed on two opposite edges of said sheet thatis electrically isolated from whole mechanical forming apparatusincluding dies which are situated at two opposite sides of said blanksheet, the process including; placing the said sheet metal workpiece asa blank on the press table; contacting said metal workpiece with atleast two electrode sets placed at two opposite edges of said blanksheet; application of current to the workpiece directly with electrodesets fed by an external current source; generating heat inside the sheetmetal workpiece to reach a certain forming temperature; stopping currentapplication to the workpiece; keeping the heated blank stretched byclamping peripheral edges of said blank by a blank holder; stamping saidheated blank sheet by actuating at least one die of the press toward;said workpiece; pulling back at least one die from the formed workpiece;removing the formed sheet workpiece from the at least one die.
 2. Theprocess as defined in claim 1, further comprising a rapid coolingprocess for stamped sheet including; keeping temperature of the dieswithin a predetermined temperature range under forming temperature ofthe workpiece by allowing heat loss from the dies during productioncycles; avoiding material shrinkage during cooling of the stampedworkpiece by means of holding the sheet between two dies at the end ofthe stamping process until a previously determined temperature at theworkpiece is reached within a certain period; providing increase inyield strength of the said sheet material by means of cooling.
 3. Theprocess as defined in claim 1 wherein at least one die includes at leastone sheet forming surface having a depression at the inner region ofsaid forming surface comprising extentions at the periphery of saiddepression to prevent contact between inner concave region of saidforming surface and facing stretched hot sheet during stretching processfor avoiding instant temperature fall at the hot sheet surface to keepformability said region of engaged hot sheet.
 4. The process as definedin claim 1 wherein at least one die is constructed from ceramicmaterial.
 5. The process as defined in claim 1 wherein said blank holderincludes at least one ceramic insert contacting with the workpiece. 6.The process as defined in claim 1 further comprising a control processwith use of a feedback control device, said control process including:measuring voltage and current rates of the current application to theworkpiece along the heating process of said blank sheet workpiece bydirect current application; calculating instantaneous electricresistance by using instantaneous voltage and current rates of thecurrent being conducted across the workpiece; calculating instantaneoustemperature rate of the workpiece by using linear correlation betweenelectric resistance and temperature; adjusting the current rate appliedto the workpiece; determining a proper actuating instant of at least onedie toward said hot blank; activating said die toward said hot blank;controlling a cooling process of stamped blank between two dies by meansof calculating a instantaneous temperature rate of the workpiece whilethe blank is being held between two dies; determining a proper instantfor removing a stamped sheet by means of moving back of at least onedie; activating said die back from the stamped sheet; removing saidformed sheet removed from the at least one die.
 7. The process asdefined in claim 1 further comprising a control process with a timerelay controlling both electric current application and stamping processin accordance with a certain time reference data for each process cycle,including; starting direct electric application to the workpiece;heating said workpiece by direct electric application within apredetermined period; clamping a hot blank by actuating a blank holderfor a predetermined period from the beginning; stamping the hot sheet byactuating at least one die upon the workpiece in a predetermined period;holding said stamped sheet between two dies for a predetermined period;moving back at least one die from the workpiece at a predetermined timefrom the beginning.
 8. The process as defined in claim 1, furthercomprising convective heat loss compensation and conductive heatexchange reducing means between hot blank sheet and the dies after atleast one die contacts with said hot blank by employing ceramic diesincluding; continiuing current application to the workpiece aftercontact occurs between at least one die and said hot sheet by avoiding ashort circuit between said sheet and at least one die by employing diesmade of nonconductive ceramic material; reducing conductive heatexchange between said dies and said hot sheet during stamping process byemploying dies made of ceramic material.
 9. The process as defined inclaim 1, further comprising providing short circuit preventing meansbetween the blank holders and said workpiece during resistance heatingwhile said workpiece is being held by the blank holders by means ofusing nonconductive ceramic coatings on the contacting surfaces of theblank holders.
 10. A process for hot bending sheet metal workpiecesperformed between at least two electrode sets placed on two oppositesides of a bending line of the blank sheet with a certain spacing amongsaid electrodes, the process including: placing the said blank intobending apparatus; contacting at least two electrode sets on twoopposite sides of the bending line of said sheet metal blank with acertain spacing among said electrodes as wide as a heating area width;starting to apply electric current to be connected across width of thebending line of said sheet workpiece via two electrode sets fed by anexternal current source; heating along said bending line with a certainwidth determined by said spacing among the two electrode sets by currentapplication at a certain rate for a certain time to reach a previouslydetermined bending temperature: holding the blank sheet on one side ofthe bending line; bending the other side of said sheet along the hotbending line by pushing with a die set on the other side of the sheet.11. An apparatus for hot forming process for sheet metal workpiecespre-heated by direct electric curret application between at least twoelectrode sets placed on two opposite edges of said sheet, the apparatusincluding: upper and lower metallic housings; said blank sheet placedbetween said upper and lower housings; at least two electrodes situatedat two opposite edges of said blank sheet to apply current to said blankto generate heat wherein; an external current source including twoconnector ends is connected to the two said electrodes; an electricalisolation between whole mechanical apparatus and current passageincluding electrodes and said blank, an upper forming die containedwithin the upper housing above said sheet workpiece; a lower housingcontaining sand lubricant mixture filled below said sheet workpiece toconnect any internal pressure toward said blank sheet; at least onehydraulic piston mounted below said lower housing to generate internalpressure inside said lower housing and fed by an external hydraulicpump; at least one movable piston rod mounted to said hydraulic pistoninserted into said sand lubricant mixture to push said sand lubricantmixture against said blank sheet to be stretched into an upper diecavity and to be formed by the die surface; a control device controllingboth heat generation inside said blank sheet by starting and stoppingcurrent application to said workpiece and controlling movement of saidat least one hydraulic piston generating internal pressure to form saidheated blank sheet by pushing said blank into the die cavity insynchronization.